Lubricant compositions for vacuum pumps

ABSTRACT

The invention pertains to a lubricant composition comprising:
         (A) at least one non-functional (per)fluoropolyether oil (PFPE oil) comprising at least one (per)fluoropolyoxyalkylene chain (chain R F );   (B) at least one cyclic phosphazene compound [compound (P)] comprising one or more cyclic moieties having formula (I) or (II) here below:       

     
       
         
         
             
             
         
       
     
     said cyclic moieties comprising, bound to one or more phosphorus atoms, at least one substituent comprising at least one (per)fluoropolyoxyalkylene chain (chain R′ F ); (C) at least one functional (per)fluoropolyether derivative (functional PFPE derivative) different from compound (P) comprising at least one (per)fluoropolyoxyalkylene chain (chain R′ F ) and at least one functional group comprising at least one monocyclic or polycyclic aromatic ring, said aromatic ring optionally containing one or more heteroatoms selected from N, O and S and one or more substituents different from hydrogen atoms.

This application claims priority to European application No. 09172280.1 filed Oct. 6, 2009, the whole content of this application being incorporated herein by reference for all purposes.

TECHNICAL FIELD

The present invention relates to (per)fluoropolyether lubricant compositions and use thereof for lubricating vacuum pumps.

BACKGROUND ART

Fluoropolyether lubricants are known in the art for being used in vacuum pumps in highly demanding applications, e.g. in the semiconductor industry, in particular for lubricating compressors and pumps handling reactive and hazardous gases, typically in rotary pumps, turbomolecular pumps, roots pumps and diffusion pumps.

Actually, major requirements for fluoropolyether lubricants to be employed for high vacuum and ultra-high vacuum applications include high thermal-oxidative stability in the presence of metals, low evaporation weight loss and high lubrication performances.

Fluoropolyether oils substantially free from volatile fractions are typically used for lubricating vacuum pumps so that back migration of the volatile oil fractions into the vacuum chamber, which is commonly due to heat generated by friction among the moving metallic parts while operating, is generally reduced.

Thus, EP 0553768 B (AUSIMONT S.P.A.) May 3, 2000 discloses use in vacuum pumps of perfluoropolyethers having at most 0.1% by weight of fractions exhibiting a molecular weight lower than or equal to 1500 and at most 1.3% by weight of fractions exhibiting a molecular weight lower than or equal to 1800, said perfluoropolyethers being typically endowed with a viscosity ranging from about 140 cSt to about 270 cSt at 20° C. and preferably complying with branched perfluoropolyethers having formulae CF₃(OCF(CF₃)CF₂)_(m)(OCF₂)_(m)OCF₃, wherein m/n is equal to or higher than 1000, and F(CF₂CF(CF₃)O)_(q)CF₂CF₃.

Nevertheless, fluoropolyether oils of the prior art, in particular linear fluoropolyether oils generally suffer from low thermal-oxidative stability in the presence of metals to be advantageously used as lubricants in high vacuum and ultra-high vacuum pumps.

EP 1454938 B (SOLVAY SOLEXIS S.P.A.) May 10, 2005 discloses linear perfluoropolyether oils comprising —CF₂CF₂O— and —CF₂O— recurring units, statistically distributed along the perfluoropolyoxyalkylene chain, in a —CF₂ CF₂O—/—CF₂O— ratio comprised between 2 and 20, which exhibit improved thermal-oxidative stability in the presence of metals with respect to known linear perfluoropolyether oils having a lower —CF₂CF₂O—/—CF₂O— ratio, typically comprised between 0.1 and 10, while maintaining a high viscosity index and a low pour point value as compared with known linear perfluoropolyether oils. The stability of the novel linear perfluoropolyether oils when used as lubricants may be further increased by the addition of perfluoropolyether-based thermal stabilizers comprising functional groups like, e.g., phosphines, phosphates, phosphazenes, benzothiazoles, triazines, amines, substituted amines, nitroderivative compounds.

The need was thus felt to provide for (per)fluoropolyether lubricants having improved thermal-oxidative stability in the presence of metals while maintaining low evaporation weight loss and high lubrication performances in terms of suitable kinematic viscosity and anti-wear and anti-rust properties, so as to be successfully used in high vacuum and ultra-high vacuum pumps by ensuring lower maintenance costs of the pumps due to a longer life-time of the lubricants and lower consumptions due to their high lubrication performances.

DISCLOSURE OF INVENTION

It is thus an object of the invention a lubricant composition comprising:

(A) at least one non-functional (per)fluoropolyether oil (non-functional PFPE oil) comprising at least one (per)fluoropolyoxyalkylene chain (chain R_(F)); (B) at least one cyclic phosphazene compound [compound (P)] comprising one or more cyclic moieties having formula (I) or (II) here below:

said cyclic moieties comprising, bound to one or more phosphorus atoms, at least one substituent comprising at least one (per)fluoropolyoxyalkylene chain (chain R′_(F)); (C) at least one functional (per)fluoropolyether derivative (functional PFPE derivative) different from compound (P) comprising at least one (per)fluoropolyoxyalkylene chain (chain R′_(F)) and at least one functional group comprising at least one monocyclic or polycyclic aromatic ring, said aromatic ring optionally containing one or more heteroatoms selected from N, O and S and one or more substituents different from hydrogen atoms.

The Applicant has surprisingly found that the lubricant composition of the invention advantageously exhibits improved thermal-oxidative stability in the presence of metals while maintaining low evaporation weight loss and high lubrication performances to be successfully used for high vacuum and ultra-high vacuum applications.

To the purpose of the present invention, “(per)fluoropolyether” is intended to denote either a fully fluorinated perfluoropolyether or a partially fluorinated fluoropolyether.

To the purpose of the present invention, “(per)fluoropolyoxyalkylene chain” is intended to denote either a fully fluorinated perfluoropolyoxyalkylene chain or a partially fluorinated fluoropolyoxyalkylene chain.

The (per)fluoropolyoxyalkylene chain (chain R_(F)) of the non-functional (per)fluoropolyether oil (non-functional PFPE oil) of the invention typically comprises one or more recurring units R^(o) having general formula —(CJJ′)_(j)—CKK′—O—, wherein J and J′, equal to or different from each other, independently represent a fluorine atom or a C₁-C₆ (per)fluoro(oxy)alkyl group, K and K′, equal to or different from each other, independently represent a hydrogen atom, a fluorine atom, a chlorine atom or a C₁-C₆ (per)fluoro(oxy)alkyl group and j is an integer comprised between 0 and 3, said recurring units being generally statistically distributed along the (per)fluoropolyoxyalkylene chain.

The non-functional PFPE oil of the invention may be a linear non-functional PFPE oil or a branched non-functional PFPE oil.

By “linear non-functional PFPE oil” it is meant a non-functional PFPE oil comprising at least one (per)fluoropolyoxyalkylene chain (chain R_(F)) as defined above, wherein J and J′ both represent a fluorine atom and K and K′, equal to or different from each other, independently represent a hydrogen atom, a fluorine atom or a chlorine atom.

By “branched non-functional PFPE oil” it is meant a non-functional PFPE oil comprising at least one (per)fluoropolyoxyalkylene chain (chain R_(F)) as defined above, wherein at least one of J, J′, K and K′ represents a C₁-C₆ (per)fluoro(oxy)alkyl group.

Representative examples of suitable non-functional PFPE oils according to the invention include, notably, the followings:

T¹-O—(C₂F₄O)_(b1′)(CF₂O)_(b2′)-T^(1′)  (1)

wherein:

-   -   T¹ and T^(1′), equal to or different from each other, are         independently selected from —CF₃, —C₂F₅ and —C₃F₇ groups;     -   b1′ and b2′, equal to or different from each other, are         independently integers ≧0 such that the b1′/b2′ ratio is         comprised between 0.1 and 5 and the (b1′+b2′) sum is comprised         between 5 and 250; should b1′ and b2′ be both different from         zero, the different recurring units are generally statistically         distributed along the perfluoropolyoxyalkylene chain. Said         products can be produced by photooxidation of C₂F₄ as reported         in U.S. Pat. No. 3,715,378 (MONTECATINI EDISON S.P.A.) Jun. 2,         1973 and subsequent treatment with fluorine as described in U.S.         Pat. No. 3,665,041 (MONTECATINI EDISON S.P.A.) May 23, 1972.

T²-O—(CF₂O)_(c1′)(CF₂CF₂O)_(c2′)(CF₂CF₂CF₂O)_(c3′)(CF₂CF₂CF₂CF₂O)_(c4′)-T^(2′)  (2)

wherein:

-   -   T² and T^(2′), equal to or different from each other, are         independently selected from —CF₃, —C₂F₅, —C₃F₇, —C₄F₉, —CF₂Cl,         —CF₂CF₂Cl groups;     -   c1, c2′, c3′ and c4′, equal to or different from each other, are         independently integers ≧0 such that the number average molecular         weight is comprised between 700 and 100000;     -   the c2′/c1′ ratio is comprised between 2 and 20;     -   the (c3′+c4′)/(c1+c2′+c3′+c4′) ratio is comprised between 0.05         and 0.2;     -   the c1′/(c1′+c2′+c3′+c4′) ratio is comprised between 0.05 and         0.4;         should at least two of c1, c2′, c3′ and c4′ be different from         zero, the different recurring units are generally statistically         distributed along the perfluoropolyoxyalkylene chain.

Said products can be produced as described in EP 1454938 B (SOLVAY SOLEXIS S.P.A.) Oct. 5, 2005.

T³-O—(CF₂CF₂O)_(d′)-T^(3′)  (3)

wherein:

-   -   T³ and T³′, equal to or different from each other, are         independently selected from —C₂F₅ and —C₃F₇ groups;     -   d′ is an integer comprised between 5 and 250.

Said products can be obtained by a method comprising fluorinating a polyethyleneoxide, e.g. with elemental fluorine, and optionally thermally fragmentating the so-obtained fluorinated polyethyleneoxide as reported in U.S. Pat. No. 4,523,039 (THE UNIVERSITY OF TEXAS) Jun. 11, 1985.

T¹-O—(CF₂CF₂C(Hal)₂O)_(e1′)—(CF₂CF₂CH₂O)_(e2′)—(CF₂CF₂CH(Hal)O)_(e3′)-T  (4)

wherein:

-   -   T¹ and T^(1′), equal to or different from each other, have the         same meaning as defined above;     -   Hal, equal or different at each occurrence, is a halogen         selected from fluorine and chlorine atoms, preferably a fluorine         atom;     -   e1′, e2′, and e3′, equal to or different from each other, are         independently integers ≧0 such that the (e1′+e2′+e3′) sum is         comprised between 5 and 250; should at least two of e1′, e2′ and         e3′ be different from zero, the different recurring units are         generally statistically distributed along the         (per)fluoropolyoxyalkylene chain.

Said products may be prepared by ring-opening polymerizing 2,2,3,3-tetrafluorooxethane in the presence of a polymerization initiator to give a polyether comprising repeating units of the formula: —CH₂CF₂CF₂O—, and optionally fluorinating and/or chlorinating said polyether, as detailed in EP 148482 B (DAIKIN INDUSTRIES LTD.) Mar. 25, 1992.

T¹-O—[CF(CF₃)CF₂O]_(f1′)(CFYO)_(f2′)-T^(1′)  (5)

wherein:

-   -   T¹ and T^(1′), equal to or different from each other, have the         same meaning as defined above;     -   Y, equal or different at each occurrence, is selected from a         fluorine atom or a —CF₃ group;     -   f1′ and f2′, equal to or different from each other, are         independently integers ≧0 such that the f1′/f2′ ratio is         comprised between 20 and 1000 and the (f1′+f2′) sum is comprised         between 5 and 250; should f1′ and f2′ be both different from         zero, the different recurring units are generally statistically         distributed along the perfluoropolyoxyalkylene chain.

Said products can be obtained by photooxidation of C₃F₆ as described in CA 786877 (MONTEDISON S.P.A.) Jun. 4, 1968 and by subsequent conversion of the end groups as described in GB 1226566 (MONTECATINI EDISON S.P.A.) Mar. 31, 1971.

T¹-O—[CF(CF₃)CF₂O]_(g1′)(C₂F₄O)_(g2′)(CFYO)_(g3′)-T^(1′)  (6)

wherein:

-   -   T¹ and T¹′, equal to or different from each other, have the same         meaning as defined above;     -   Y, equal or different at each occurrence, has the same meaning         as defined above;     -   g1′, g2′ and g3′, equal to or different from each other, are         independently integers ≧0 such that the (g1′+g2′+g3′) sum is         comprised between 5 and 250; should at least two of g1′, g2′ and         g3′ be different from zero, the different recurring units are         generally statistically distributed along the         perfluoropolyoxyalkylene chain.

Said products can be manufactured by photooxidation of a mixture of C₃F₆ and C₂F₄ and subsequent treatment with fluorine as described in U.S. Pat. No. 3,665,041 (MONTECATINI EDISON S.P.A.) May 23, 1972.

C₃F₇O—[CF(CF₃)CF₂O]_(h′)-T³  (7)

wherein:

-   -   T³ has the same meaning as defined above;     -   h′ is an integer comprised between 5 and 250.

Said products can be prepared by ionic hexafluoropropylene epoxide oligomerization and subsequent treatment with fluorine as described in U.S. Pat. No. 3,242,218 (E. I. DU PONT DE NEMOURS AND CO.) Mar. 22, 1966.

{C₃F₇O—[CF(CF₃)CF₂O]_(i′)—CF(CF₃)—}₂  (8)

wherein:

-   -   i′ is an integer comprised between 2 and 250.

Said products can be obtained by ionic telomerization of the hexafluoropropylene epoxide and subsequent photochemical dimerization as reported in U.S. Pat. No. 3,214,478 (E. I. DU PONT DE NEMOURS AND CO.) Oct. 26, 1965.

R¹ _(f)—{C(CF₃)₂—O—[C(R² _(f))₂]_(j1′)C(R² _(f))₂—O}_(j2′)—R¹ _(f)  (9)

wherein:

-   -   R¹ _(f), equal or different at each occurrence, is a C₁-C₆         perfluoroalkyl group;     -   R² _(f), equal or different at each occurrence, is selected from         a fluorine atom and a C₁-C₆ perfluoroalkyl group;     -   j1′ is equal to 1 or 2;     -   j2′ is an integer comprised between 5 and 250.

Said products can be produced by the copolymerization of hexafluoroacetone with an oxygen-containing cyclic comonomer selected from ethylene oxide, propylene oxide, epoxy-butane and/or trimethylene oxide (oxethane) or substituted derivatives thereof and subsequent perfluorination of the resulting copolymer, as detailed in patent application WO 87/00538 (LAGOW ET AL.) Jan. 1, 1987.

Preferred non-functional PFPE oils suitable for the purpose of the invention include, notably, the followings:

(i) non-functional PFPE oils complying with formula (2) as described above, wherein:

-   -   T² and T²′ are —CF₃ groups;     -   c1′, c2′, c3′ and c4′, equal to or different from each other,         are independently integers >0 such that the number average         molecular weight is comprised between 1000 and 20000;     -   the c2′/c1′ ratio is comprised between 2 and 5;     -   the (c3′+c4′)/(c1′+c2′+c3′+c4′) ratio is comprised between 0.07         and 0.2;     -   the c1′/(c1′+c2′+c3′+c4′) ratio is comprised between 0.1 and         0.3;         the different recurring units being generally statistically         distributed along the perfluoropolyoxyalkylene chain;         (ii) non-functional PFPE oils commercially available from Solvay         Solexis S.p.A. under the trade name FOMBLIN® (type M, Z or Y),         said oils generally comprising at least one oil complying with         either of formulae here below:

CF₃—[(OCF₂CF₂)_(m)—(OCF₂)_(n)]—OCF₃

m+n=40-180; m/n=0.5-2

CF₃[(OCF(CF₃)CF₂)_(p)—(OCF₂)_(q)]—OCF₃

p+q=8-45; p/q=20-1000

(iii) non-functional PFPE oils commercially available from Daikin under the trade name DEMNUM®, said oils generally comprising at least one oil complying with formula here below:

F—(CF₂CF₂CF₂O)_(n)—(CF₂CF₂CH₂O)_(j)—CF₂CF₃

j=0 or integer >0; n+j=10-150

(iv) non-functional PFPE oils commercially available from Du Pont de Nemours under the trade name KRYTOX®, said oils generally comprising at least one low-molecular weight, fluorine end-capped, homopolymer of hexafluoropropylene epoxide complying with formula here below:

F—(CF(CF₃)CF₂O)_(n)—CF₂CF₃

n=10-60

More preferred non-functional PFPE oils suitable for the purpose of the invention include, notably, non-functional PFPE oils complying with formula (2) as described hereinabove and non-functional PFPE oils commercially available from Solvay Solexis S.p.A. under the trade name FOMBLIN® (type M, Z or Y) as described hereinabove.

A mixture of more than one non-functional PFPE oil as defined above may also be used in the lubricant composition of the invention.

Preferred mixtures of more than one non-functional PFPE oil as defined above include, notably, mixtures of at least one linear non-functional PFPE oil as defined above with at least one branched non-functional PFPE oil as defined above. Representative examples of suitable mixtures of at least one linear non-functional PFPE oil as defined above with at least one branched non-functional PFPE oil as defined above include, notably, those commercially available from Solvay Solexis S.p.A. under the trade name FOMBLIN® W.

The (per)fluoropolyoxyalkylene chain (chain R′_(F)) of the cyclic phosphazene compound [compound (P)] of the invention typically comprises one or more recurring units R¹ having general formula —(CF₂)_(j)—CFK—O—, wherein K is selected from a fluorine atom and a C₁-C₅ (per)fluoro(oxy)alkyl group and j is an integer comprised between 0 and 3, said recurring units being generally statistically distributed along the (per)fluoropolyoxyalkylene chain.

According to a first embodiment of the invention, the compound (P) complies with formula (I-A), (I-A bis) or (II-A) here below:

wherein:

-   -   R_(f) is a (per)fluoropolyoxyalkylene chain comprising one or         more recurring units having formula         —CF₂O(CF₂CF₂O)_(p′)(CF₂O)_(q′)CF₂—, wherein p′ and q′ are         integers such that the number average molecular weight of the         chain is comprised between 1000 and 2000, the q′/p′ ratio being         comprised between 0.2 and 2;     -   R′_(f) is a (per)fluoropolyoxyalkylene chain comprising one or         more recurring units having formula —(C₃F₆O)_(q)(CF(CF₃)O)_(r)—,         wherein q and r are integers such that the number average         molecular weight of the chain is comprised between 500 and 700,         the q/r ratio being ≧2.

Compounds (P) complying with formula (I-A), (I-A bis) or (II-A) as described in this first embodiment of the invention are notably disclosed in EP 1336614 A (SOLVAY SOLEXIS S.P.A.) Aug. 20, 2003.

According to a second embodiment of the invention, the compound (P) complies with formula (I-B) or (II-B) here below:

wherein:

-   -   R″_(f) and R′″_(f), equal to or different from each other,         independently represent a (per)fluoropolyoxyalkylene chain         comprising one or more recurring units selected from:         (i) —CFYO—, wherein Y is a fluorine atom or a —CF₃ group,         (ii) —CF₂CFYO—, wherein Y has the same meaning as defined above,         (iii) —CF₂CF₂CF₂O—,

(iv) —CF₂CF₂CF₂CF₂O—,

said recurring units being generally statistically distributed along the (per)fluoropolyoxyalkylene chain;

-   -   Z and Z′, equal to or different from each other, independently         represent a polar group of formula —O⁻M⁺, wherein M is selected         from hydrogen, a monovalent metal, preferably an alkaline metal,         and an ammonium radical having formula —NR₁R₂R₃R₄, wherein each         of R₁, R₂, R₃, R₄ is, independently, a hydrogen atom or a C₁-C₁₂         hydrocarbon group, optionally fluorinated, or a polar group of         formula —O⁻)₂M′²⁺, wherein M′ is a divalent metal, preferably an         alkaline earth metal;     -   n_(Z) is an integer comprised between 1 and 3, preferably equal         to 1;     -   n_(Z′) is an integer comprised between 1 and 4, preferably equal         to 1;     -   n_(f) is an integer such that n_(Z)+n_(f) is equal to 6;     -   n_(f′) is an integer such that n_(Z′)+n_(f′) is equal to 8.

The compound (P) of this second embodiment of the invention preferably complies with formula (I-B) or (II-B) here above, wherein R″_(f)O— and R′″_(f)O— are selected from:

T⁴-O—(CF₂CF₂O)_(p′)(CF₂O)_(q′)-J-  (a)

wherein:

-   -   T⁴ is selected from —CF₃, —CF₂CF₃, —CF₂CF₂CF₃,         —(CF₂)_(f)—CF₂(T₁) and —(CF₂)_(f)—CF(T₁)(T₂), wherein f is 0 or         1, T₁ and T₂, equal to or different from each other, are         independently selected from chlorine, bromine and hydrogen         atoms;     -   J is selected from groups having formulae         —CE′(Y)CE₂—O—(CH₂CH₂O)_(s1)— and —CE′YO(CH₂CH₂O)_(s2)—, wherein         E and E′, equal to or different from each other, are         independently selected from a fluorine atom, a hydrogen atom and         a —CF₃ group, E being preferably a hydrogen atom and E′ being         preferably selected from a fluorine atom and a —CF₃ group, Y is         selected from a fluorine atom and a —CF₃ group, s1 and s2, equal         to or different from each other, are integers comprised between         0 and 5;     -   p′ and q′ are numbers such that the (p′+q′) sum is comprised         between 1 and 100 and the q′/p′ ratio is comprised between 0.1         and 10;

T⁵-O—(CF₂CF(CF₃)O)_(r′)—(CF₂CF₂O)_(s′)—(CFYO)_(t′)-J-  (b)

wherein:

-   -   T⁵ is selected from —CF₃, —CFYCF₃, —CF(Y)CF(Y)CF₃,         —(CFY)_(f)—CFY(T′₁) and —(CFY)_(f)—CY(T′₁)(T′₂), wherein f is 0         or 1, T′₁ and T′₂, equal to or different from each other, are         independently selected from chlorine, bromine and hydrogen         atoms;     -   Y, equal or different at each occurrence, has the same meaning         as defined above;     -   J has the same meaning as defined above;     -   r′, s′ and t′ are numbers such that the (r′+s′+t′) sum is         comprised between 1 and 100, the t′/(r′+s′) ratio being         comprised between 0.01 and 0.1;

T⁵-(OCFY)_(t1)″(OC₃F₆)_(u1″)—OR*_(f)O—(C₃F₆O)_(u2″)(CFYO)_(t2″)-J-  (c)

wherein:

-   -   T⁵ has the same meaning as defined above;     -   Y, equal or different at each occurrence, has the same meaning         as defined above;     -   J has the same meaning as defined above;     -   R*_(f) is a C₁-C₈ perfluoroalkylene group;     -   t1″, u1″, t2″, u2″ are numbers such that the (t1′u1′t2′u2″) sum         is comprised between 1 and 100, the (t1′t2″)/(u1′u2″) ratio         being comprised between 0.01 and 0.1;

T⁴-CF₂CF₂O—(CF₂—(CF₂)_(x′)CF₂O)_(v′)-J-  (d)

wherein:

-   -   T⁴ has the same meaning as defined above;     -   J has the same meaning as defined above;     -   x′ is equal to 1 or 2;     -   v′ is a number comprised between 1 and 100;

T⁴-CF₂CH₂—(OCF₂CF₂CH₂)_(w′)—OR*_(f)O—(CH₂CF₂CF₂O)_(w′)-J-  (e)

wherein:

-   -   T⁴ has the same meaning as defined above;     -   J has the same meaning as defined above;     -   R*_(f) has the same meaning as defined above;     -   w′ is a number comprised between 1 and 100.

The compound (P) of this second embodiment of the invention more preferably complies with formula (I-B*) or (II-B*) here below:

wherein:

-   -   Z, Z′, n_(z), n_(z′), n_(f) and n_(f′), have the same meaning as         defined above;     -   R″*_(f)O— and R′″*_(f)O—, equal to or different from each other,         represent a fluoropolyoxyalkylene chain having formula:

T*-O—(CF₂CF₂O)_(p)*(CF₂O)_(q)*-CF₂—CH₂—O—(CH₂CH₂O)_(s)*—

wherein:

-   -   T* is selected from —CF₃, —CF₂CF₃ and —CF₂CF₂CF₃ groups;     -   p* and q* are numbers such that the (p*+q*) sum is comprised         between 1 and 50 and the q*/p* ratio is comprised between 0.1         and 10;     -   s* is a number comprised between 0 and 5.

The compound (P) of this second embodiment of the invention most preferably complies with formula (III-B) or (III-B bis) here below:

wherein:

-   -   M and M′ have the same meaning as defined above, M being         preferably an alkaline metal selected from Li, Na and K or an         ammonium radical having formula —NR₁R₂R₃R₄, wherein each of R₁,         R₂, R₃, R₄ is, independently, a hydrogen atom or a C₁-C₁₂         hydrocarbon group, preferably R₁═R₂=R₃═R₄=n-butyl, and M′ being         preferably an alkaline earth metal selected from Ca and Mg, more         preferably M′ being Ca;     -   each of p*i (i=1 to 5) and q*i (i=1 to 5) is independently an         integer 0 such that the (p*i+q*i) sum is comprised between 2 and         25 and the q*i/p*i ratio is comprised between 0.1 and 10.

Compounds (P) complying with formula (I-B) or (II-B) as described in this second embodiment of the invention are notably disclosed in WO 2008/000706 (SOLVAY SOLEXIS S.P.A.) Jan. 3, 2008.

According to a third embodiment of the invention, the compound (P) typically complies with formula (IV-C) here below:

wherein:

-   -   A, equal or different at each occurrence, is selected from:         (i) -QR groups, wherein Q is a divalent group selected from —O—,         —S—, —NR₁— and —NH—NH—, wherein R₁ is a hydrogen atom or a C₁-C₄         alkyl group, and R is a C₆-C₁₂ aryl group, preferably a phenyl         group, optionally substituted by one or more, preferably 1 or 2         groups selected from C₁-C₄ alkyl groups, —OR₂—, —NR₃R₄, —NO₂,         fluorine and chlorine atoms, wherein R₂, R₃ and R₄ are hydrogen         atoms or C₁-C₄ alkyl groups;         (ii) R_(F)CH₂—O—(CH₂CH₂O)_(s)—, wherein R_(F) is a         (per)fluoropolyoxyalkylene chain (chain R′_(F)) as defined above         and s is equal to 0, 1 or 2;     -   m is equal to 3 or 4;     -   n is equal to 0.

Compounds (P) complying with formula (IV-C) as described in this third embodiment of the invention are notably disclosed in EP 0597369 A (AUSIMONT S.P.A.) Sep. 16, 1998.

Preferred lubricant compositions of the invention comprise one or more compounds (P) according to the first and/or the second embodiment of the invention as detailed above.

More preferred lubricant compositions of the invention comprise one or more compounds (P) according to the second embodiment of the invention as detailed above.

Most preferred lubricant compositions of the invention comprise one or more compounds (P) complying with either of formulae (III-B) or (III-B bis) as described above.

The functional (per)fluoropolyether derivative (functional PFPE derivative) of the invention comprises at least one (per)fluoropolyoxyalkylene chain (chain R′_(F)) as defined above and at least one functional group typically comprising at least one ether, thioether, ester or amine group comprising at least one monocyclic or polycyclic aromatic ring, said aromatic ring optionally containing one or more heteroatoms selected from N, O and S and one or more substituents different from hydrogen atoms.

The functional (per)fluoropolyether derivative (functional PFPE derivative) of the invention preferably complies with formula (V) here below:

T-(CFW₁)_(p1)—O—R_(F)—(CFW₂)_(p1′)-T′  (V)

wherein:

-   -   R_(F) is a (per)fluoropolyoxyalkylene chain (chain R′_(F)) as         defined above;     -   T and T′, equal to or different from each other, are selected         from:         i) functional end-groups comprising at least one ether,         thioether, ester or amine group comprising at least one         monocyclic or polycyclic aromatic ring, said aromatic ring         optionally containing one or more heteroatoms selected from N, O         and S and one or more substituents different from hydrogen         atoms,         ii) non-functional end-groups selected from a fluorine atom, a         chlorine atom and a C₁-C₃ (per)fluoroalkyl group optionally         containing one or more chlorine atoms,         with the proviso that at least one of T and T′ is a functional         end-group as defined hereinabove;     -   W₁ and W₂, equal to or different from each other, independently         represent a fluorine atom or a —CF₃ group;     -   p1 and p1′, equal to or different from each other, are         independently integers comprised between 1 and 3, preferably         being equal to 1 when W₁ and/or W₂ are —CF₃ groups.

The functional PFPE derivative of the invention more preferably complies with formula (VI) here below:

T″-CF₂O(CF₂CF₂O)_(m)—(CF₂O)_(n)—CF₂-T′″  (VI)

wherein:

-   -   T″ and T′″, equal to or different from each other, are selected         from:         i) functional end-groups selected from those having formulae         —CH₂-A-Y, —CH(CF₃)-A-Y, —CHW′—B—(CH₂CH₂O)_(m′)—Y and —COOY,         wherein:         (a′) A is selected from oxygen and sulphur atoms,         (b′) B is selected from an oxygen atom, a sulphur atom and a —NH         group,         (c′) W′ is selected from a hydrogen atom and a —CF₃ group,         (d′) m′ is an integer comprised between 0 and 6,         (e′) Y is selected from phenyl, pyridine, triazine, benzoxazole,         benzothiazole and benzopyrazine groups optionally substituted         with one or more substituents different from hydrogen atoms,         ii) non-functional end-groups selected from a fluorine atom, a         chlorine atom, a —CF₃ group, a —C₂F₅ group and a —(CF₂)_(n)Cl         group, wherein n is an integer comprised between 1 and 3, with         the proviso that at least one of T″ and T′″ is a functional         end-group as defined hereinabove;     -   m* and n* are integers such that the number average molecular         weight of the functional PFPE derivative is comprised between         400 and 10000, the m*/n* ratio being comprised between 0.1 and         10.

Preferred lubricant compositions of the invention comprise one or more functional PFPE derivatives selected from:

-   -   functional PFPE derivatives complying with formula (V) as         described above wherein at least one of T and T′ is a functional         end-group comprising at least one ether or thiether group         comprising at least one pyridine group optionally substituted         with one or more substituents different from hydrogen atoms,         notably disclosed in EP 1479753 A (SOLVAY SOLEXIS S.P.A.) Nov.         24, 2004;     -   functional PFPE derivatives complying with formula (V) as         described above wherein at least one of T and T′ is a functional         end-group comprising at least one ether or ester group         comprising at least one phenyl group optionally substituted with         one or more substituents different from hydrogen atoms, notably         disclosed in EP 1354932 B (SOLVAY SOLEXIS S.P.A.) Jun. 14, 2006;     -   functional PFPE derivatives complying with formula (V) as         described above wherein at least one of T and T′ is a functional         end-group comprising at least one ether, thioether or amine         group comprising at least one 1,3,5-triazine group optionally         substituted with one or more substituents different from         hydrogen atoms, notably disclosed in EP 1712580 B (SOLVAY         SOLEXIS S.P.A.) Dec. 10, 2008;     -   functional PFPE derivatives complying with formula (V) as         described above wherein at least one of T and T′ is a functional         end-group comprising at least one ether or thioether group         comprising at least one benzoxazole, benzothiazole or         benzopyrazine group optionally substituted with one or more         substituents different from hydrogen atoms, notably disclosed in         EP 1659164 A (SOLVAY SOLEXIS S.P.A.) May 24, 2006.

Most preferred lubricant compositions of the invention comprise one or more functional PFPE derivatives complying with formula (VI) as described above wherein both T″ and T′″ are functional end-groups as defined above (bifunctional PFPE derivatives).

Non-limitative examples of suitable bifunctional PFPE derivatives include, notably, the followings:

-   -   bifunctional PFPE derivatives complying with formula here below:

wherein m* and n* are integers such that the number average molecular weight of the bifunctional PFPE derivative is comprised between 1000 and 6000, preferably between 2000 and 4000, the m/n ratio being comprised between 0.1 and 5;

-   -   bifunctional PFPE derivatives commercially available from Solvay         Solexis S.p.A. under the trade name FOMBLIN® DA 410, said         bifunctional PFPE derivatives complying with formula here below:

wherein m* and n* are integers such that the number average molecular weight of the bifunctional PFPE derivative is comprised between 1000 and 6000, preferably between 2000 and 4000, the m/n ratio being comprised between 0.1 and 5.

Very good results have been obtained with lubricant compositions comprising one or more bifunctional PFPE derivatives complying with formula here below:

wherein m* and n* are integers such that the number average molecular weight of the bifunctional PFPE derivative is comprised between 1000 and 6000, preferably between 2000 and 4000, the m/n ratio being comprised between 0.1 and 5.

The lubricant composition of the invention typically comprises at least one cyclic phosphazene compound [compound (P)] as defined above in an amount of at least 0.1% by weight, preferably at least 0.2% by weight, more preferably at least 0.3% by weight with respect to the weight of the non-functional PFPE oil.

The lubricant composition of the invention typically comprises at least one cyclic phosphazene compound [compound (P)] as defined above in an amount of at most 15% by weight, preferably at most 10% by weight, more preferably at most 5% by weight with respect to the weight of the non-functional PFPE oil.

The lubricant composition of the invention typically comprises at least one functional PFPE derivative different from compound (P) as defined above in an amount of at least 0.5% by weight, preferably at least 1% by weight, more preferably at least 2% by weight with respect to the weight of the non-functional PFPE oil.

The lubricant composition of the invention typically comprises at least one functional PFPE derivative different from compound (P) as defined above in an amount of at most 15% by weight, preferably at most 10% by weight, more preferably at most 6% by weight with respect to the weight of the non-functional PFPE oil.

Very good results have been obtained with lubricant compositions comprising at least one cyclic phosphazene compound [compound (P)] in an amount ranging from 0.3% to 5% by weight with respect to the weight of the non-functional PFPE oil and at least one functional PFPE derivative different from compound (P) as defined above in an amount ranging from 2% to 6% by weight with respect to the weight of the non-functional PFPE oil.

The lubricant composition of the invention typically has a kinematic viscosity (η) ranging from 100 to 350, preferably from 120 to 300, as measured at 20° C. by ASTM D445.

The lubricant composition can also further comprise other additives notably including one or more anti-rust additives, oxidation inhibitors, antifoam agents, anti-wear additives and anti-scuff additives. Representative examples of anti-rust additives which can be advantageously added to the lubricant composition of the invention include, notably, functional PFPEs commercially available from Solvay Solexis S.p.A. under the trade names FOMBLIN® DA305, FOMBLIN® DA306 and FOMBLIN® DA308.

Another object of the invention is use of the lubricant composition as described above for lubricating vacuum pumps, in particular high vacuum and ultra-high vacuum pumps, especially in the semiconductor industry where high-purity lubricants are typically required.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The invention will be now described in more detail with reference to the following examples whose purpose is merely illustrative and not limitative of the scope of the invention.

Raw Materials

The non-functional PFPE oil (1) is a linear non-functional PFPE oil having formula CF₃O—(CF₂O)_(n)(CF₂CF₂O)_(m)(CF₂CF₂CF₂O)_(r)(CF₂CF₂CF₂CF₂O)_(s)—CF₃, wherein m, n, r and s are integers such that the number average molecular weight is about 11000, the m/n ratio being about 2.7.

The non-functional PFPE oil (2) is a FOMBLIN® M15 linear non-functional PFPE oil having formula CF₃O—(CF₂CF₂O)_(m)(CF₂O)_(n)—CF₃, wherein m and n are integers such that the number average molecular weight is about 10600, the m/n ratio being about 0.9.

The non-functional PFPE oil (3) is a FOMBLIN® M30 linear non-functional

PFPE oil having formula CF₃O—(CF₂CF₂O)_(m)(CF₂O)_(n)—CF₃, wherein m and n are integers such that the number average molecular weight is about 13000, the m/n ratio being about 0.9.

The non-functional PFPE oil (4) is a FOMBLIN® Y25/6 branched non-functional PFPE oil having formula

CF₃O—(CF(CF₃)CF₂O)_(p)(CF₂O)_(q)—CF₃, wherein p and q are integers such that the number average molecular weight is about 3800.

The compound (P1) is a cyclic phosphazene compound having formula

wherein p*i (i=1 to 5) and q*i (i=1 to 5) indexes are integers such that the number average molecular weight is about 8300.

The functional PFPE derivative (1) has formula

wherein m and n are integers such that the number average molecular weight is about 3800, the m/n ratio being about 0.9.

Thermal-Oxidative Stability Test Method

Thermal-oxidative stability tests were conducted on a sintered metal bearing of Fe—Cu—Sn—C type.

The sintered metal bearing of Fe—Cu—Sn—C type was impregnated with the lubricant composition of the invention at 30° C., under vacuum (7 mbar), for 25 minutes. The sintered metal bearing so treated was then allowed to drain for about 15 minutes, so that the amount of lubricant composition impregnated in the sintered metal bearing was about 0.05 g.

The sintered metal bearing so impregnated was placed in a ventilated oven set at 200° C. and weighted every 48-72 hours.

Weight loss of the lubricant composition was checked as a function of time.

An induction time was measured until complete degradation of the lubricant composition.

Tribological Test Method

Sliding wear tests were conducted with a ball-on-disk tribometer at 140° C. during 6 hours under the following operating conditions:

-   -   ball diameter: 10 mm;     -   applied load: 200 N;     -   amplitude of oscillation: 1 mm;     -   frequency of oscillation: 50 Hz.

The wear volume (V) on the disk [mm³] has been calculated as follows:

$V = \frac{\left( {d\; 1} \right)^{2} \cdot \left( {d\; 2} \right)^{2} \cdot 3.14}{64\; R}$

wherein:

-   -   d¹ represents the diagonal of the disk wear parallel to the         oscillating motion;     -   d2 represents the diagonal of the disk wear perpendicular to the         oscillating motion;     -   R represents the ball radius (5 mm).

Example 1

A lubricant composition was prepared by mixing non-functional PFPE oil (3) with 0.5% by weight of non-functional PFPE oil (3) of compound (P1) and 5% by weight of non-functional PFPE oil (3) of functional PFPE derivative (1).

The results for the thermal-oxidative stability test of said lubricant composition are set forth in Table 1 here below.

Example 1C Comparative

A composition was prepared by mixing non-functional PFPE oil (3) with 0.5% by weight of non-functional PFPE oil (3) of compound (P1).

The results for the thermal-oxidative stability test of said composition are set forth in Table 1 here below.

Example 2C Comparative

A composition was prepared by mixing non-functional PFPE oil (3) with 5% by weight of non-functional PFPE oil (3) of functional PFPE derivative (1).

The results for the thermal-oxidative stability test of said composition are set forth in Table 1 here below.

TABLE 1 Non-functional Induction time Example PFPE oil [hours] 1 PFPE oil (3) 1104 1C PFPE oil (3) 816 2C PFPE oil (3) 744

The thermal-oxidative stability tests performed with the lubricant composition of Example 1 of the invention showed a substantial increase of the induction time to degradation with respect to compositions of comparative Examples 1C and 2C, that is to say that the lubricant composition of the invention showed improved thermal-oxidative stability with respect to compositions comprising at least one non-functional PFPE oil as defined above and either at least one compound (P) as defined above or at least one functional PFPE derivative as defined above.

Example 2

A lubricant composition was prepared by mixing non-functional PFPE oil (1) with 3% by weight of non-functional PFPE oil (1) of compound (P1) and 3% by weight of non-functional PFPE oil (1) of functional PFPE derivative (1).

The results for the tribological test and the evaporating constant at 150° C. [(mm³ of evaporated lubricant/mm² of exposed surface area)·h] ([*]) of said lubricant composition are set forth in Table 2 here below.

Example 3

A lubricant composition was prepared by mixing non-functional PFPE oil (2) with 3% by weight of non-functional PFPE oil (2) of compound (P1) and 3% by weight of non-functional PFPE oil (2) of functional PFPE derivative (1).

The results for the tribological test and the evaporating constant at 150° C. [(mm³ of evaporated lubricant/mm² of exposed surface area)·h] ([*]) of said lubricant composition are set forth in Table 2 here below.

Example 4

A lubricant composition was prepared by mixing non-functional PFPE oil (4) with 3% by weight of non-functional PFPE oil (4) of compound (P1) and 3% by weight of non-functional PFPE oil (4) of functional PFPE derivative (1).

The results for the tribological test and the evaporating constant at 150° C. [(mm³ of evaporated lubricant/mm² of exposed surface area)·h] ([*]) of said lubricant composition are set forth in Table 2 here below.

Example 3C Comparative

A composition was prepared by mixing non-functional PFPE oil (2) with 3% by weight of non-functional PFPE oil (2) of compound (P1).

The results for the tribological test and the evaporating constant at 150° C. [(mm³ of evaporated lubricant/mm² of exposed surface area)·h] ([*]) of said composition are set forth in Table 2 here below.

The thermal-oxidative stability tests performed with the composition of Example 3C showed a substantial decrease of the induction time to degradation with respect to the lubricant composition of Example 4 of the invention.

Example 4C Comparative

A composition was prepared by mixing non-functional PFPE oil (2) with 3% by weight of non-functional PFPE oil (2) of functional PFPE derivative (1).

The results for the tribological test and the evaporating constant at 150° C. [(mm³ of evaporated lubricant/mm² of exposed surface area)·h] ([*]) of said composition are set forth in Table 2 here below.

The thermal-oxidative stability tests performed with the composition of Example 4C showed a substantial decrease of the induction time to degradation with respect to the lubricant composition of Example 4 of the invention.

TABLE 2 Non-functional Wear volume Evaporating constant Example PFPE oil [mm3] [*] 2 PFPE oil (1) 0.075 4.89 · 10⁻³ 3 PFPE oil (2) 0.071 3.77 · 10⁻³ 4 PFPE oil (4) 0.067 1.13 · 10⁻² 3C PFPE oil (2) 0.069 2.83 · 10⁻³ 4C PFPE oil (2) 0.087 1.36 · 10⁻³

Substantially low wear volumes and low evaporating constants were measured for the lubricant compositions of Examples 2, 3 and 4 of the invention, that is to say that the lubricant compositions of the invention exhibited substantially high lubrication performances in terms of anti-wear properties and low evaporation weight loss. In particular, the tribological tests performed with the lubricant composition of Example 3 of the invention showed that substantially low or even slightly decreased wear volumes were measured with respect to compositions of comparative Examples 3C and 4C, that is to say that the lubricant composition of the invention showed good or slightly improved anti-wear properties with respect to compositions comprising at least one non-functional PFPE oil as defined above and either at least one compound (P) as defined above or at least one functional PFPE derivative as defined above. 

1. A lubricant composition comprising: (A) at least one non-functional (per)fluoropolyether oil (non-functional PFPE oil) comprising at least one (per)fluoropolyoxyalkylene chain (chain R_(F)); (B) at least one cyclic phosphazene compound [compound (P)] comprising one or more cyclic moieties having formula (I) or (II) here below:

said cyclic moieties comprising, bound to one or more phosphorus atoms, at least one substituent comprising at least one (per)fluoropolyoxyalkylene chain (chain R′_(F)); and (C) at least one functional (per)fluoropolyether derivative (functional PFPE derivative) different from compound (P) comprising at least one (per)fluoropolyoxyalkylene chain (chain R′_(F)) and at least one functional group comprising at least one monocyclic or polycyclic aromatic ring, said aromatic ring optionally containing one or more heteroatoms selected from N, O and S and one or more substituents different from hydrogen atoms.
 2. The lubricant composition of claim 1, wherein the non-functional PFPE oil is selected from the group consisting of followings: T¹-O—(C₂F₄O)_(b1′)(CF₂O)_(b2′)-T^(1′)  (1) wherein: T¹ and T^(1′), equal to or different from each other, are independently selected from —CF₃, —C₂F₅ and —C₃F₇ groups; b1′ and b2′, equal to or different from each other, are independently integers ≧0 such that the b1′/b2′ ratio is comprised between 0.1 and 5 and the (b1′+b2′) sum is comprised between 5 and 250; should b1′ and b2′ be both different from zero, the different recurring units are generally statistically distributed along the perfluoropolyoxyalkylene chain. T²-O—(CF₂O)_(c1′)(CF₂CF₂O)_(c2′(CF) ₂CF₂CF₂O)_(c3′)(CF₂CF₂CF₂CF₂O)_(c4′)-T^(2′)  (2) wherein: T² and T^(2′), equal to or different from each other, are independently selected from —CF₃, —C₂F₅, —C₃F₇, —C₄F₉, —CF₂C1, —CF₂CF₂C1 groups; c1′, c2′, c3′ and c4′, equal to or different from each other, are independently integers ≧0 such that the number average molecular weight is comprised between 700 and 100000; the c2′/c1′ ratio is comprised between 2 and 20; the (c3′+c4′)/(c1′+c2′+c3′+c4′) ratio is comprised between 0.05 and 0.2; the c1′/(c1′+c2′+c3′+c4′) ratio is comprised between 0.05 and 0.4; should at least two of c1′, c2′, c3′ and c4′ be different from zero, the different recurring units are generally statistically distributed along the perfluoropolyoxyalkylene chain. T³-O—(CF₂CF₂O)_(d′)-T^(3′)  (3) wherein: T³ and T³′, equal to or different from each other, are independently selected from —C₂F₅ and —C₃F₇ groups; d′ is an integer comprised between 5 and
 250. T¹-O—(CF₂CF₂C(Hal)₂O)_(e1′—(CF) ₂CF₂CH₂O)_(e2′—(CF) ₂CF₂CH(Hal)O)_(e3′)-T^(1′)  (4) wherein: T¹ and T^(1′), equal to or different from each other, have the same meaning as defined above; Hal, equal or different at each occurrence, is a halogen selected from fluorine and chlorine atoms, preferably a fluorine atom; e1′, e2′, and e3′, equal to or different from each other, are independently integers ≧0 such that the (e1′+e2′+e3′) sum is comprised between 5 and 250; should at least two of e1′, e2′ and e3′ be different from zero, the different recurring units are generally statistically distributed along the (per)fluoropolyoxyalkylene chain. T¹-O—[CF(CF₃)CF₂O]_(f1′)(CFYO)_(f2′-T) ^(1′)  (5) wherein: T¹ and T^(1′), equal to or different from each other, have the same meaning as defined above; Y, equal or different at each occurrence, is selected from a fluorine atom or a —CF₃ group; f1′ and f2′, equal to or different from each other, are independently integers ≧0 such that the f1′/f2′ ratio is comprised between 20 and 1000 and the (f1′+f2′) sum is comprised between 5 and 250; should f1′ and f2′ be both different from zero, the different recurring units are generally statistically distributed along the perfluoropolyoxyalkylene chain. T¹-O—[CF(CF₃)CF₂O]_(g1′)(C₂F₄O)_(g2′)(CFYO)_(g3′)-T¹′  (6) wherein: T¹ and T¹′, equal to or different from each other, have the same meaning as defined above; Y, equal or different at each occurrence, has the same meaning as defined above; g1′, g2′ and g3′, equal to or different from each other, are independently integers ≧0 such that the (g1′+g2′+g3′) sum is comprised between 5 and 250; should at least two of g1′, g2′ and g3′ be different from zero, the different recurring units are generally statistically distributed along the perfluoropolyoxyalkylene chain. C₃F₇O—[CF(CF₃)CF₂O]_(h′)-T³  (7) wherein: T³ has the same meaning as defined above; h′ is an integer comprised between 5 and
 250. {C₃F₇O—[CF(CF₃)CF₂O]_(i′)—CF(CF₃)—}₂  (8) wherein: i′ is an integer comprised between 2 and 250, and R¹ _(f)—{C(CF₃)₂—O—[C(R² _(f))₂]_(j1′)C(R² _(f))₂—O}_(j2′)—R¹ _(f)  (9) wherein: R¹ _(f), equal or different at each occurrence, is a C₁-C₆ perfluoroalkyl group; R² _(f), equal or different at each occurrence, is selected from a fluorine atom and a C₁-C₆ perfluoroalkyl group; j1′ is equal to 1 or 2; j2′ is an integer comprised between 5 and
 250. 3. The lubricant composition of claim 1, wherein a mixture of more than one non-functional PFPE oil is used.
 4. The lubricant composition of anyone of claim 1, wherein the cyclic phosphazene compound [compound (P)] complies with formula (I-B) or (II-B) here below:

wherein: R″_(f) and R′″_(f), equal to or different from each other, independently represent a (per)fluoropolyoxyalkylene chain comprising one or more recurring units selected from the group consisting of: (i) —CFYO—, wherein Y is a fluorine atom or a —CF₃ group, (ii) —CF₂CFYO—, wherein Y has the same meaning as defined above, (iii) —CF₂CF₂CF₂O—, and (iv) —CF₂CF₂CF₂CF₂O—, said recurring units being generally statistically distributed along the (per)fluoropolyoxyalkylene chain; Z and Z′, equal to or different from each other, independently represent a polar group of formula —O⁻M⁺, wherein M is selected from hydrogen, a monovalent metal, preferably an alkaline metal, and an ammonium radical having formula —NR₁R₂R₃R₄, wherein each of R₁, R₂, R₃, R₄ is, independently, a hydrogen atom or a C₁-C₁₂ hydrocarbon group, optionally fluorinated, or a polar group of formula —O⁻)₂M′²⁺, wherein M′ is a divalent metal, preferably an alkaline earth metal; n_(Z) is an integer comprised between 1 and 3, preferably equal to 1; n_(Z′) is an integer comprised between 1 and 4, preferably equal to 1; n_(f) is an integer such that n_(Z)+n_(f) is equal to 6; n_(f′) is an integer such that n_(Z′+n) _(f′) is equal to
 8. 5. The lubricant composition of claim 4, wherein the cyclic phosphazene compound [compound (P)] complies with formula (I-B*) or (II-B*) here below:

wherein: Z, Z′, n_(z), n_(z′), n_(f) and n_(f′), have the same meaning as defined in claim 4; R″*_(f)O— and R′″*_(f)O—, equal to or different from each other, represent a (per)fluoropolyoxyalkylene chain having formula: T*-O—(CF₂CF₂O)_(p)*(CF₂O)_(q)—CF₂—CH₂—O—(CH₂CH₂O)_(s)*— wherein: T* is selected from —CF₃, —CF₂CF₃ and —CF₂CF₂CF₃ groups; p* and q* are numbers such that the (p*+q*) sum is comprised between 1 and 50 and the q*/p* ratio is comprised between 0.1 and 10; s* is a number comprised between 0 and
 5. 6. The lubricant composition of claim 4, wherein the cyclic phosphazene compound [compound (P)] complies with formula (III-B) or (III-B bis) here below:

wherein: M and M′ have the same meaning as defined in claim 4, M being preferably an alkaline metal selected from Li, Na and K or an ammonium radical having formula —NR₁R₂R₃R₄, wherein each of R₁, R₂, R₃, R₄ is, independently, a hydrogen atom or a C₁-C₁₂ hydrocarbon group, preferably R₁═R₂=R₃═R₄=n-butyl, and M′ being preferably an alkaline earth metal selected from Ca and Mg, more preferably M′ being Ca; each of p*i (i=1 to 5) and q*i (i=1 to 5) is independently an integer ≧0 such that the (p*i+q*i) sum is comprised between 2 and 25 and the q*i/p*i ratio is comprised between 0.1 and
 10. 7. The lubricant composition of anyone of claim 1, wherein the functional PFPE derivative complies with formula (V) here below: T-(CFW₁)_(p1)—O—R_(F)—(CFW₂)_(p1′)-T′  (V) wherein: R_(F) is a (per)fluoropolyoxyalkylene chain (chain R′_(F)); T and T′, equal to or different from each other, are selected from: i) functional end-groups comprising at least one ether, thioether, ester or amine group comprising at least one monocyclic or polycyclic aromatic ring, said aromatic ring optionally containing one or more heteroatoms selected from N, O and S and one or more substituents different from hydrogen atoms, ii) non-functional end-groups selected from the group consisting of a fluorine atom, a chlorine atom and a C₁-C₃ (per)fluoroalkyl group optionally containing one or more chlorine atoms, with the proviso that at least one of T and T′ is a functional end-group as defined hereinabove; W₁ and W₂, equal to or different from each other, independently represent a fluorine atom or a —CF₃ group; p1 and p1′, equal to or different from each other, are independently integers comprised between 1 and 3, preferably being equal to 1 when W₁ and/or W₂ are —CF₃ groups.
 8. The lubricant composition of claim 7, wherein the functional PFPE derivative complies with formula (VI) here below: T″-CF₂O(CF₂CF₂O)_(m)*(CF₂O)_(n)*CF₂-T′″  (VI) wherein: T″ and T′″, equal to or different from each other, are selected from: i) functional end-groups selected from those having formulae —CH₂-A-Y, —CH(CF₃)-A-Y, —CHW′—B—(CH₂CH₂O)_(m′)—Y and —COOY, wherein: (a′) A is selected from oxygen and sulphur atoms, (b′) B is selected from an oxygen atom, a sulphur atom and a —NH group, (c′) W′ is selected from a hydrogen atom and a —CF₃ group, (d′) m′ is an integer comprised between 0 and 6, (e′) Y is selected from phenyl, pyridine, triazine, benzoxazole, benzothiazole and benzopyrazine groups optionally substituted with one or more substituents different from hydrogen atoms, ii) non-functional end-groups selected from the group consisting of a fluorine atom, a chlorine atom, a —CF₃ group, a —C₂F₅ group and a —(CF₂)_(n)Cl group, wherein n is an integer comprised between 1 and 3, with the proviso that at least one of T″ and T′″ is a functional end-group as defined hereinabove; m* and n* are integers such that the number average molecular weight of the functional PFPE derivative is comprised between 400 and 10000, the m*/n* ratio being comprised between 0.1 and
 10. 9. The lubricant composition of anyone of claim 1, wherein at least one cyclic phosphazene compound [compound (P)] is comprised in an amount ranging from 0.3% to 5% by weight with respect to the weight of the non-functional PFPE oil and at least one functional PFPE derivative different from compound (P) is comprised in an amount ranging from 2% to 6% by weight with respect to the weight of the non-functional PFPE oil.
 10. A method for lubricating vacuum pumps comprising the use of the lubricant composition of claim
 1. 